Underfill Flow Numerical Simulation for Achieving Board Level Low Cost and High Reliability of Mobile Devices

Underfill is used for improving the mechanical reliability of large Ball Grid Array (BGA) and Wafer Level Chip Scale Packaging (WLCSP) chips in mobile devices. In order to reduce cost, by reducing the amount of underfill usage, it is recommended to selectively cover the solder joints with underfill that are prone to crack under mechanical loads. In this research, a numerical model was developed to predict the underfill flow, so that the flow can cover the selected solder joints. Effects of dispensing process parameters on the area of flow are studied. The simulation was implemented with ANSYS FLUENT. The Volume of Fluid (VOF) technique was used to track the flow front. A Multiple velocity inlets boundary condition with time-dependent velocity setting were proposed to deal with the movement of the dispense nozzle. Underfill dispensing experiments were done to verify the accuracy of the simulation. The results show that by adjusting the process parameters, the area of underfill flow can be adjusted to cover the selected solder joints, and the area of flow around the chip can be controlled to keep away from the devices around the chip that are not allowed to be covered by the underfill.